An old toothbrush
Preferably stiffer paper
The first mistake I did was to put in way too much water when mixing the watercolours. These resulted into giant blobs of coloured water drops on the paper.
Hubs showed Bubs how to hold the toothbrush upside down and flick them with his fingers. Bubs gave it a cautious few tries.
Bubs started to get the hang of it and had so much fun messing around with the colours. He experimented with shaking out the wet blobs and then tilting the paper so the colours streamed down.
Non stop flicking. First with one hand then both at the same time. Be warned, this is a very messy activity and the colours go everywhere. We'd to stop a few times so I could wash the toothbrush in between colours to avoid everything from turning a yucky shade of brown.
The end result :)